U-Th-Pb and Rb-Sr systematics of Allende and U-Th-Pb systematics of Orgueil

U-Th-Pb systematics study of Allende inclusions showed that U, Th and Sr concentrations in Ca, Al (pyroxene)-rich chondrules and white and pinkish-white aggregate separates of Allende are five to ten times higher than those of the matrix, whereas Mg (olivine)-rich chondrules have U and Th concentrations about twice as high as the matrix. Th concentrations are extremely high in white aggregates and in pinkish-white (spinel-rich) aggregates while U and Sr concentrations in white aggregates are more than twice as high as those in pinkish-white aggregates. Large enrichment of these refractory elements in the white aggregates indicates that they contain high-temperature condensates from the solar nebula. The Pb concentrations in the inclusions are less than half of those in the whole rock and matrix, indicating that the matrix is a lower-temperature condensate. The isotopic composition of lead in the matrix is less radiogenic than that of the whole meteorite, whereas lead in Ca- and Al-rich chondrules and aggregates is extremely radiogenic. The ²⁰⁶Pb/²⁰⁴Pb ratio reaches as high as 55.9 in a white aggregate separate. The lead of Mg-rich chondrules is moderately radiogenic and the ²⁰⁶Pb/²⁰⁴Pb ratio ranges from 18 to 26. A striking linear relationship exists among leads in the chondrules, aggregates and matrix on the ²⁰⁷Pb/²⁰⁴Pb vs ²⁰⁴Pb/²⁰⁴Pb plot. The slope of the best fit line is 0.6188 ± 0.0016, yielding an isochron age of 4553 ± 4 m.y. The regression line passes through primordial lead values obtained from Canyon Diablo troilite. The data, when corrected for Canyon Diablo troilite Pb and plotted on a U-Pb concordia diagram, show that the pink and white aggregates and the Ca-Al-rich and Mg-rich inclusions have excess Pb and define a chord which intersects the concordia curve at 4548 ± 25 m.y. and 107 ± 70 m.y. The intercepts might correspond to the agglomeration age of the meteorite and a time of probably later disturbance, respectively. The matrix and some chondrules which contain less radiogenic lead did, however, not fit on the chord. The Rb-Sr data of Allende did not define an isochron suggesting that the Rb-Sr system was also disturbed by a later event, as suggested by the U-Pb concordia data. The lowest observed ⁸⁷Sr/⁸⁶Sr ratio in Allende inclusions is similar to the initial ratio of the Angra dos Reis achondrite (Papanastassiou, Thesis, 1970).The initial Pb isotopic composition of Orgueil calculated by a single-stage evolution model is more radiogenic than that of Canyon Diablo troilite. To reconcile the U-Pb data of Orgueil and Allende, we propose that the initial lead isotopic composition of the carbonaceous chondrites was slightly different from that of Canyon Diablo troilite Pb.     

Metallogeny and lead isotopic composition of base metal sulfide deposits in the Rappen area,northern Sweden

Early Proterozoic volcanic and sedimentary rocks of the Rappen district in northern Sweden were deposited at a destructive plate margin to the south of the Archaean craton of the western Baltic Shield. The volcano-sedimentary suite was intruded by two generations of early Proterozoic granites at ca. 1.89–1.85 Ga and ca.1.82–1.78 Ga, respectively, and metamorphosed at upper amphibolite facies conditions. Small stratabound iron, copper, and zinc deposits occur in felsic to mafic tuffs and arkosic sediments. Small deposits of molybdenum, tungsten, and uranium formed during the emplacement of the younger granites. The lead isotopic compositions of sulfide trace lead from the various deposits are highly heterogeneous. In the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram they fall on mixing arrays between little evolved early Proterozoic lead and highly radiogenic Caledonian lead. The least radiogenic lead isotopic compositions from the various deposits have a wide range of ²⁰⁷Pb/²⁰⁴Pb ratios and thus indicate variable involvement of Archaean crustal lead in the Proterozoic deposits. Deposits hosted by siliciclastic rocks have higher ²⁰⁷Pb/²⁰⁴Pb ratios than deposits hosted in mafic to felsic tuffites. The lead isotopic heterogeneity suggests that the lead in the various deposits was locally derived and, furthermore, that the sedimentary rocks in part originated from the Archaean craton to the north. Lead mixing arrays in the ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb diagram demonstrate that in Paleozoic time radiogenic lead was mobilized and transported in the basement. Source ages calculated from the mixing arrays (ca.1.9 Ga and ca.1.8 Ga) correspond to the age of the Early Proterozoic volcanism and metamorphism respectively. One group of deposits includes lead from at least three sources and illustrates that radiogenic lead was multiply mobilized and transported in the Proterozoic basement. It occurs in deposits that occur in zones that became permeable during the reactivations of the basement.     

Genesis of the South Zhuguang Uranium Ore Field,South China: Pb Isotopic Compositions and Mineralization Ages

U–Pb isotopic analyses indicate that ores from the South Zhuguang uranium ore field, south China, have high common (non‐radiogenic) Pb contents, with variable and relatively radiogenic initial Pb contents. The U–Pb isochron method was used to date these ores, with plots of ²⁰⁸Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb versus ²⁰⁶Pb/²⁰⁴Pb being used to identify sample suites with similar initial Pb isotopic ratios and to normalize variable initial Pb isotopic ratios. The resulting U–Pb isochrons indicate two substages of uranium mineralization at ~57 and 52 Ma, with a later hydrothermal reformation at ~49 Ma, which homogenized Pb isotopic compositions. Initial Pb isotopic systematics indicate that the ore‐forming fluid was characterized by high ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios and low ²⁰⁸Pb/²⁰⁴Pb ratios, suggesting that the ore‐forming fluid was sourced from Cretaceous–Paleogene red‐bed basins, rather than from magma or the mantle, with consideration of mineralization ages.     

Differences in lead isotope composition in the stratiform McArthur zinc-lead-silver deposit

Lead isotopic composition and uranium and lead concentrations have been determined for galena, sphalerite, pyrite and acetic acid soluble material from the McArthur area in order to test the hypothesis of a dual sulphur source suggested by the sulphur isotope data of Smith and Croxford (Sulphur isotope ratios in the McArthur lead-zinc-silver deposit, Nature Phys. Sci. 245, 10–12 (1973)). Galena, sphalerite and the acetic acid washes from the McArthur deposit have uniform isotopic ratios (²⁰⁶Pb/²⁰⁴Pb, 16.07–16.15; ²⁰⁷Pb/²⁰⁴Pb, 15.37–15.47; ²⁰⁸Pb/²⁰⁴Pb, 35.57–35.89) consistent with other conformable ore deposits, whereas the ratios for pyrite are variable and quite radiogenic (²⁰⁶Pb/²⁰⁴Pb, 16.24–16.49; ²⁰⁷Pb/²⁰⁴Pb, 15.42–15.58; ²⁰⁸Pb/²⁰⁴Pb, 35.82–36.98). Acid washes where dolomite is a major dissolved phase are also radiogenic. The lead in the pyrite appears to have been derived from at least two sources: the less radiogenic lead coming from an exhalative source as for galena and sphalerite and the more radiogenic lead probably being leached from the country rocks. It is proposed that analysis of pyrite for isotopic composition and concentration of lead could be used as an indicator for similar types of deposits in this area.     

Isotope characterization of lead in galena from ore deposits of the Aysén Region, southern Chile

Lead isotope analyses of galena from five ore deposits and six prospects in the Aysén region of southern Chile are reported. Most of the deposits are either low sulfidation epithermal gold–silver veins or skarn and manto deposits; the majority are either suspected to be, or dated as, Late Jurassic to mid-Cretaceous. Galena lead isotope data for most of the deposits from southern Chile cluster near the “orogene” within a “plumbotectonic” model framework. Average values (²⁰⁶Pb/²⁰⁴Pb=18.53, ²⁰⁷Pb/²⁰⁴Pb=15.63, and ²⁰⁸Pb/²⁰⁴Pb=38.50) are near Jurassic to Cretaceous model ages on the “orogene” curve of Zartman and Doe (1981) and the second-stage curve of Stacey and Kramers (1975) on a ²⁰⁶Pb/²⁰⁴Pb versus ²⁰⁷Pb/²⁰⁴Pb plot. These model ages are compatible with absolute ages as currently known. The elongate trends in the general cluster indicate mainly an orogenic model fit, suggesting variable mixing of lead from different sources, mainly model upper crust and lesser model mantle and lower crust reservoirs. Galena lead associated with one deposit (El Faldeo) is relatively radiogenic, and lies near a Jurassic age on the “upper crustal” curve of Zartman and Doe (1981), which is compatible with the Ar/Ar age of the deposit. Galena lead isotope clusters define three main groups of deposits. These three groups appear to be related to three mineralizing events, dated by K–Ar and Ar/Ar, in the Late Jurassic (group 3), and in the Early and mid-Cretaceous (groups 1 and 2 respectively). Averages for group 1, the northern group including El Toqui and Katerfeld, are ²⁰⁶Pb/²⁰⁴Pb=18.51, ²⁰⁷Pb/²⁰⁴Pb=15.62, ²⁰⁸Pb/²⁰⁴Pb=38.48. Averages for group 2, the southern group with Fachinal and Mina Silva, are ²⁰⁶Pb/²⁰⁴Pb = 18.56, ²⁰⁷Pb/²⁰⁴Pb=15.63, ²⁰⁸Pb/²⁰⁴Pb=38.52. Averages for group 3, the southernmost group with the El Faldeo, Lago Chacabuco and Lago Cochrane prospects, are ²⁰⁶Pb/²⁰⁴Pb=18.83, ²⁰⁷Pb/²⁰⁴Pb=15.65, ²⁰⁸Pb/²⁰⁴Pb=38.63. The Cretaceous deposits (groups 1 and 2) contain orogene-type lead that becomes increasingly radiogenic southward. Lead from the Late Jurassic deposits (group 3) appears to reflect mixing of orogene lead with highly radiogenic lead. The observed linear array of lead in group 3 probably reflects mixing of orogene lead with highly radiogenic lead, which was likely extracted by selective leaching of mineralizing hydrothermal solutions from the metamorphic basement. Received: 10 July 1999 / Accepted: 15 July 2000     

Thallium and lead in the Allende C3V carbonaceous chondrite. A study of the matrix phase

Tl and Pb isotopic abundances have been measured in various phases from Allende and the distribution and siting of these elements in the matrix phase investigated. Matrix fractions, prepared by sieving, sedimentation, magnetic separation and acid etching, were further characterised by X-ray diffraction and SEM. Tl concentrations range from 1 ppb in coarse grained inclusions to 1560 ppb in the acid-etched carbon residues and from 32 ppb to 194 ppb in the 16 matrix fractions. Pb concentrations which range from 0.1 ppm to 3.1 ppm, are enhanced in magnetic phases and depleted in Allende pentlandite relative to the whole meteorite. The Tl-²⁰⁴Pb abundance diagram is described near the origin by the inclusions and chondrules and extends through sulphide to the non-magnetic and magnetic matrix fractions. Abundances in the finest grained matrix fractions form a linear trend which passes through the origin and the Orgueil and Murchison whole meteorite data. The deviation of magnetic matrix fractions from the above linear trend is probably related to the presence of an intimate association of an awaruite-sulphide-carbon species. 92% of the Tl in the carbon residues, which is released on hydrolysis with HCl, probably resides in an organic host molecule(s) on the macromolecular carbonaceous material, whereas the surface film of organics on the matrix grains shows no apparent enrichment of Tl.Tl and ²⁰⁴Pb abundances revealed an inverse correlation with grain size and a distribution within the grains rather than as a surface layer, is indicated. Constraints placed by the data on the formational environment of the matrix grains are considered. Interstellar shock heating and rapid radiative cooling is a possible mechanism for the establishment of the observed inverse correlation of volatile abundance with grain size. Matrix data do not lend support for a recent redistribution of lead as an explanation for the apparent excess Pb in Allende. The apparent initial Pb isotopic compositions of the matrix fractions are heterogeneous and not attributable to terrestrial contamination. The very magnetic fractions have high abundances of Pb and the least radiogenic apparent initial Pb compositions, whereas the non-magnetic fractions have lower Pb abundances but more radiogenic apparent initial Pb compositions. The data also indicate that use of the predicted Tl-²⁰⁴Pb cosmothermometer, to infer accretion temperatures, is apparently not valid for individual phases of Allende.     

Strontium and Lead Isotopic Study of the Carbonate‐hosted Xujiashan Antimony Deposit from Hubei Province,South China: Implications for its Origin

The Xujiashan antimony deposit is hosted by marine carbonates of the Upper Sinian Doushantuo and Dengying Formations in Hubei Province, South China. Our Sr isotopic data from pre‐ and syn‐mineralization calcites that host the mineralization show that the pre‐mineralization calcite displays a narrow range of ⁸⁷Sr/⁸⁶Sr ratios (0.7096 to 0.7097), similar to the ratios of the Sinian seawater, and high Sr concentrations (2645 to 8174 ppm). In contrast, the syn‐mineralization calcite exhibits low Sr concentrations (785 to 2563 ppm) and high ⁸⁷Sr/⁸⁶Sr ratios (0.7109 to 0.7154), which is interpreted as the result of addition of radiogenic strontium during the antimony mineralization. The study of Sr isotopes suggests that their Sr component to the pre‐mineralization calcite derived directly from the host rocks (i.e. the Sinian marine carbonates), while radiogenic ⁸⁷Sr for the syn‐mineralization calcite derived from the underlying Mesoproterozoic Lengjiaxi Group basement through hydrothermal fluid circulation along the major fault that hosts the mineralization. The Pb isotopic ratios of stibnite are subdivided into two groups (Group A and Group B), Group A is characterized by higher radiogenic lead, with ²⁰⁶Pb/²⁰⁴Pb = 18.874 to 19.288, ²⁰⁷Pb/²⁰⁴Pb = 15.708 to 15.805, and ²⁰⁸Pb/²⁰⁴Pb = 38.642 to 39.001. Group B shows lower lead isotope ratios (²⁰⁶Pb/²⁰⁴Pb = 17.882 to 18.171, ²⁰⁷Pb/²⁰⁴Pb = 15.555 to 15.686, and ²⁰⁸Pb/²⁰⁴Pb = 37.950 to 38.340). The single‐stage model ages of Group A are mainly negative or slightly positive values (‐258 to 3 Ma), while those of Group B range from 636 to 392 Ma, with an average of 495 ± 65 Ma. In addition, there are positive linear correlations among Pb isotopic ratios. These results suggest that the lead of Group A stibnite was mainly derived from the Sinian marine carbonates, and that of Group B stibnite from the underlying Lengjiaxi Group basement. This conclusion is consistent with the results of the Sr isotopes. These results indicate that the Xujiashan deposit is not syngenetic sedimentary and in situ reworked origin as previously considered. The metal (mainly Sb) of this deposit was not only derived from the Sinian host rocks, but also partly derived from the underlying Mesoproterozoic Lengjiaxi Group basement.     

Sulfur, Carbon and Lead Isotope Studies of the Dajing Polymetallic Deposit in Linxi County, Inner Mongolia, China – Implication for Metallogenic Elements from Hypomagmatic Source

Abstract: The Dajing Cu‐polymetallic ore deposit in Linxi county, Inner Mongolia Autonomous Region, China, is economically a valuable Cu–Sn–Ag–Zn–Pb deposit in the southern section of the Da Hinggan metallogenic province. For the analyzed 23 samples of sulfide minerals, including chalcopyrite, pyrite, sphalerite and galena, the δ³⁴S values range from –1.8 to +3.8 % with an average of +0.65 %. The narrow distributions of the δ³⁴S values with +1 % peak value, including the published data, and the δ¹³C values around –5 % indicate that the sulfur and carbon of the hydrothermal fluids are derived from a hypomagmatic source, and exclude the possibility that the hosted strata, i.e., the Upper Permian Linxi Formation, provided certain amounts of sulfur and carbon. The ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of sulfide ores range respectively within 18.257‐18.368, 15.476‐15.609, and 37.916‐38.355 with the model ages of 122–209 Ma. The black shale, however, contains higher radiogenic lead with the ²⁰⁶Pb/²⁰⁴Pb ratios of 18.473‐20.156, differing from the ores. However, the ²⁰⁶Pb/²⁰⁴Pb, ²⁰⁷Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios of the ore, basaltic porphyrite and feldspar leads are similar, and lie on the same lines in the diagrams of ²⁰⁸Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb. The fact that these mixing lines are composed of the two end members, the mantle and orogenic belt, strongly supports that all the metallogenic elements were carried by the hypomagma mixing the matters of the mantle and orogenic belt prior to the Mesozoic. Therefore, the Dajing ore deposit is a typical mag–matic–hydrothermal vein type ore deposit associated with subvolcanic rocks.     

A Correction Approach to the Common Pb Contribution to ^207Pb／^206Pb Ratios Obtained by the Zircon Evaporation Technique

An approach is recommended for the correction of common Pb contribution to ^207Pb/^206Pb ages obtained by the zircon evaporation technique.A comparison with that by Cocherie et al.(1992)shows that two approaches yield similar results in the ^207Pb*/^206Pb* ratios.But when using the new approach,only two errors of the measured ^204Pb/^206Pb and ^207Pb/^207Pb ratios are introduced to the calculated ^207Pb^*/^206Pb^* ratios.     

Lead isotopes of the carbonate-hosted Kabwe, Tsumeb, and Kipushi Pb-Zn-Cu sulphide deposits in relation to Pan African orogenesis in the Damaran-Lufilian Fold Belt of Central Africa

Lead isotope ratios of galena from the carbonate-hosted massive sulphide deposits of Kabwe (Pb-Zn) and Tsumeb (Pb-Zn-Cu) in Zambia and Namibia, respectively, have been measured and found to be homogeneous and characteristic of upper crustal source rocks. Kabwe galena has average isotope ratios of ^206/204Pb = 17.997 ± 0.007, ^207/204Pb = 15.713 ± 0.010 and ^208/204Pb = 38.410 ± 0.033. Tsumeb galena has slightly higher ^206/204Pb (18.112 ± 0.035) and slightly lower ^207/204Pb (15.674 ± 0.016) and ^208/204Pb (38.276 ± 0.073) ratios than Kabwe galena. The isotopic differences are attributed to local differences in the age and composition of the respective source rocks for Kabwe and Tsumeb. The homogeneity of the ore lead in the two epigenetic deposits suggests lead sources of uniform isotopic composition or, alternatively, thorough mixing of lead derived from sources with relatively similar isotopic compositions. Both deposits have relatively high ²³⁸U/²⁰⁴Pb ratios of 10.31 and 10.09 for Kabwe and Tsumeb galenas, respectively. These isotope ratios are considered to be typical of the upper continental crust in the Damaran-Lufilian orogenic belt, as also indicated by basement rocks and Cu-Co sulphides in stratiform Katangan metasediments which have a mean μ-value of 10.25 ± 0.12 in the Copperbelt region of Zambia and the Democratic Republic of Congo (formerly Zaire). The ²³²Th/²⁰⁴Pb isotope ratios of 43.08 and 40.42 for Kabwe and Tsumeb suggest Th-enriched source regions with ²³²Th/²³⁵U (κ-values) of 4.18 and 4.01, respectively. Model isotopic ages determined for the Kabwe (680 Ma) and Tsumeb (530 Ma) deposits indicate that the timing of the mineralisation was probably related to phases of orogenic activity associated with the Pan-African Lufilian and Damaran orogenies, respectively. Galena from the carbonate-hosted Kipushi Cu-Pb-Zn massive sulphide deposit in the Congo also has homogeneous lead isotope ratios, but its isotopic composition is comparable to that of the average global lead evolution curve for conformable massive sulphide deposits. The μ (9.84) and κ (3.69) values indicate a significant mantle component, and the isotopic age of the Kipushi deposit (456 Ma) suggests that the emplacement of the mineralisation was related to a post-tectonic phase of igneous activity in the Lufilian belt. The isotope ratios (^206/204Pb, ^207/204Pb, ^208/204Pb) of the three deposits are markedly different from the heterogeneous lead ratios of the Katangan Cu-Co stratiform mineralisation of the Copperbelt as well as those of the volcanogenic Nampundwe massive pyrite deposit in the Zambezi belt which typically define radiogenic linear trends on lead-lead plots. The host-rock dolomite of the Kabwe deposit also has homogeneous lead isotope ratios identical to the ore galena. This observation indicates contamination of the Kabwe Dolomite Formation with ore lead during mineralisation. Received: 8 September 1997 / Accepted: 21 August 1998     

Plumbotectonic aspects of polymetallic vein mineralization in Paleozoic sediments and Proterozoic basement of Moravia (Czech Republic)

A regional isotopic study of Pb and S in hydrothermal galenas and U–Pb and S in potential source rocks was carried out for part of Moravia, Czech Republic. Two major generations of veins, (syn-) Variscan and post-Variscan, are defined based on the Pb-isotope system together with structural constraints (local structures and regional trends). The Pb-isotopic compositions of galena plot in two distinct populations with outliers in ²⁰⁶Pb/²⁰⁴Pb–²⁰⁷Pb/²⁰⁴Pb space. Galena from veins hosted in greywackes provides a cluster with the lowest Pb–Pb ratios: ²⁰⁶Pb/²⁰⁴Pb = 18.15–18.27, ²⁰⁷Pb/²⁰⁴Pb = 15.59–15.61, ²⁰⁸Pb/²⁰⁴Pb = 38.11–38.23. Those hosted in both limestones and greywackes provide the second cluster: ²⁰⁶Pb/²⁰⁴Pb = 18.37–18.44, ²⁰⁷Pb/²⁰⁴Pb = 15.60–15.63, ²⁰⁸Pb/²⁰⁴Pb = 38.14–38.32. These clusters suggest model Pb ages as Early Carboniferous and Triassic–Jurassic, the latter associated with MVT-like deposits. Two samples from veins hosted in Proterozoic rocks lie outside the two clusters: in metagranitoid (²⁰⁶Pb/²⁰⁴Pb = 18.55, ²⁰⁷Pb/²⁰⁴Pb = 15.64, ²⁰⁸Pb/²⁰⁴Pb = 38.29) and in orthogneiss (²⁰⁶Pb/²⁰⁴Pb = 18.79, ²⁰⁷Pb/²⁰⁴Pb = 15.73, ²⁰⁸Pb/²⁰⁴Pb = 38.54). The results from these two samples suggest an interaction of mineralizing fluids with the radiogenic Pb-rich source (basement?). The values of δ³⁴S suggest the Paleozoic host rocks (mostly ?6.7 to +5.2‰ CDT) as the source of S for hydrothermal sulfides (mostly ?4.8 to +2.5‰ CDT). U–Pb data and Pb isotope evolutionary curves indicate that Late Devonian and Early Carboniferous sediments, especially siliciclastics, are the general dominant contributor of Pb for galena mineralization developed in sedimentary rocks. Plumbotectonic mixing occurred, it is deduced, only between the lower and the upper crust (the latter involving Proterozoic basement containing heterogeneous radiogenic Pb), without any significant input from the mantle. It is concluded that in the Moravo–Silesian and Rhenohercynian zones (including proximal districts in Poland) lead and sulfur have been mobilized from the adjacent rocks during multiple hydrothermal events in processes that are remarkably comparable in timing, geochemistry of fluids and nature of sources.     

Determination of lead isotopic composition of a grain from the allende carbonaceous chondrite by LAMMA

The attempt of this work is to use the LAMMA, a newly developed modern micro-beam analytical technique, to determine the lead isotopic composition of some grains picked out from the white aggregate in the Allende (C3V) carbonaceous chondrite. The experimental results show that the Pb/Pb model age of the Allende calculated on the basis of the²⁰⁶Pb/²⁰⁷Pb ratio is 4.56 ±0.008 b. y., just in agreement with the values reported in literature. The LAMMA provides a new and powerful tool in the analytical arsenal dealing with trace elements and isotopic compositions, due to its high sensitivity, rapid performence and good space resolution.     

A lead isotope study of mineralization in the Saudi Arabian Shield

New lead isotope data are presented for some late Precambrian and early Paleozoic vein and massive sulfide deposits in the Arabian Shield. Using the Stacey Kramers (1975) model for lead isotope evolution, isochron model ages range between 720 m.y. and 420 m.y. Most of the massive sulfide deposits in the region formed before 680 m.y. ago, during evolution of the shield. Vein type mineralization of higher lead content occurred during the Pan African event about 550 m.y. ago and continued through the Najd period of extensive faulting in the shield that ended about 530 m.y. ago. Late post-tectonic metamorphism may have been responsible for vein deposits that have model ages less than 500 m.y. Alternatively some of these younger model ages may be too low due to the mineralizing fluids acquiring radiogenic lead from appreciably older local crustal rocks at the time of ore formation.The low²⁰⁷Pb/²⁰⁴Pb ratios found for the deposits in the main part of the shield and for those in north-eastern Egypt, indicate that the Arabian craton was formed in an oceanic crustal environment during the late Precambrian. Involvement of older, upper-crustal material in the formation of the ore deposits in this part of the shield is precluded by their low²⁰⁷Pb/²⁰⁴Pb and²⁰⁸Pb/²⁰⁴Pb characteristics.In the eastern part of the shield, east of longitude 44°20E towards the Al Amar-Idsas fault region, lead data are quite different. They exhibit a linear²⁰⁷Pb/²⁰⁴Pb-²⁰⁶Pb/²⁰⁴Pb relationship together with distinctly higher²⁰⁸Pb/²⁰⁴Pb characteristics. These data imply the existence of lower crustal rocks of early Proterozoic age that apparently have underthrust the shield rocks from the east. If most of the samples we have analyzed from this easterly region were mineralized 530 m.y. ago, then the age of the older continental rocks is 2,100±300 m.y. (2).The presence of upper crustal rocks, possibly also of early Proterozoic age, is indicated by galena data from Hailan in South Yemen and also from near Muscat in Oman. These data are the first to indicate such old continental material in these regions.     

Concentration, isotopic composition, and sources of lead in Southern Ocean air during 1999/2000, measured at the Cape Grim Baseline Air Pollution Station, Tasmania

Southern Ocean aerosols were collected at the Cape Grim Baseline Air Pollution Station from onshore air under baseline conditions between February 1999 and April 2000. Thermal ionization techniques (TIMS) and isotope dilution mass spectrometry (IDMS) were used to measure the isotopic composition and concentration of lead in the air giving concentrations as low as 0.6 ± 0.1 pg · m⁻³. Air collected under baseline conditions for 12 months (May 1999-April 2000) yielded an overall lead concentration of 11.0 ± 0.2 pg · m⁻³ and isotopic composition of ²⁰⁶Pb/²⁰⁷Pb = 1.154, ²⁰⁸Pb/²⁰⁷Pb = 2.387 and ²⁰⁶Pb/²⁰⁴Pb = 17.93. The range in isotopic ratios was consistent with the mixing of lead from major population centers in the Southern Hemisphere in the mid to high latitudes, except for the presence of highly radiogenic lead in some samples. Contributions from radiogenic lead of up to ∼0.8% were observed. Three periods with the highest percentage contribution of radiogenic lead (>0.5%) were investigated in more detail, and 4-d back-trajectories and radon concentrations were used to help identify the sources. The sources are probably associated with the mining and processing of uranium rich ores in southern Africa and possibly South Australia.     

Pb isotopic constraints on the formation of the Dikulushi Cu–Pb–Zn–Ag mineralisation, Kundelungu Plateau (Democratic Republic of Congo)

Base metal–Ag mineralisation at Dikulushi and in other deposits on the Kundelungu Plateau (Democratic Republic of Congo) developed during two episodes. Subeconomic Cu–Pb–Zn–Fe polysulphide ores were generated during the Lufilian Orogeny (c. 520 Ma ago) in a set of E–W- and NE–SW-oriented faults. Their lead has a relatively unradiogenic and internally inhomogeneous isotopic composition (²⁰⁶Pb/²⁰⁴Pb = 18.07–18.49), most likely generated by mixing of Pb from isotopically heterogeneous clastic sources. These sulphides were remobilised and enriched after the Lufilian Orogeny, along reactivated and newly formed NE–SW-oriented faults into a chalcocite-dominated Cu–Ag mineralisation of high economic interest. The chalcocite samples contain only trace amounts of lead and show mostly radiogenic Pb isotope signatures that fall along a linear trend in the ²⁰⁷Pb/²⁰⁴Pb vs. ²⁰⁶Pb/²⁰⁴Pb diagram (²⁰⁶Pb/²⁰⁴Pb = 18.66–23.65; ²⁰⁷Pb/²⁰⁴Pb = 15.72–16.02). These anomalous characteristics reflect a two-stage evolution involving admixture of both radiogenic lead and uranium during a young fluid event possibly c. 100 Ma ago. The Pb isotope systematics of local host rocks to mineralisation also indicate some comparable young disturbance of their U–Th–Pb systems, related to the same event. They could have provided Pb with sufficiently radiogenic compositions that was added to less radiogenic Pb remobilised from precursor Cu–Pb–Zn–Fe polysulphides, whereas the U most likely originated from external sources. Local metal sources are also suggested by the ²⁰⁸Pb/²⁰⁴Pb–²⁰⁶Pb/²⁰⁴Pb systematics of combined ore and rock lead, which indicate a pronounced and diversified lithological control of the immediate host rocks on the chalcocite-dominated Cu–Ag ores. The Pb isotope systematics of polysulphide mineralisation on the Kundelungu Plateau clearly record a diachronous evolution.     

Caledonian upgrading of Proterozoic low-grade base-metal deposits in Northern Sweden

Summary Proterozoic sulfide deposits within the basement of northern Sweden have lead isotopic compositions that fall on a mixing line in the²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb diagram. These deposits contain a highly radiogenic Phanerozoic lead component that was leached from the Proterozoic basement at around 0.4 Ga during the Caledonian orogeny. Within the Proterozoic deposits, the less radiogenic lead isotopic compositions occur in undeformed and little deformed sections, while the more radiogenic lead isotopic compositions are observed along fault, fracture, and shear zones. These zones with radiogenic Phanerozoic lead also have higher contents of lead, zinc, and gold, respectively, than the other parts of the deposits, which suggests that these metals were introduced together with the radiogenic lead at a much later event than the metals in the unaltered Proterozoic deposit. The Proterozoic deposits acted as traps for metal additions along Caledonian reactivated fault and shear zones in the Proterozoic basement.

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Kaledonische Metallanreicherung in niedrighaltigen proterozoischen Buntmetallerzen in Nordschweden

Zusammenfassung Proterozoische Sulfidvererzungen im proterozoischen Grundgebirge weisen Bleiisotopenzusammensetzungen auf, die auf eine Mischungslinie im²⁰⁶Pb/²⁰⁴Pb-²⁰⁷Pb/²⁰⁴Pb Diagramm fallen. Die proterozoischen Vererzungen enthalten eine Komponente radiogenen Bleis, welches im Verlaufe der kaledonischen Orogenese aus dem proterozoischen Grundgebirge ausgelaugt wurde. Wenig oder nicht deformierte Abschnitte in den proterozoischen Sulfidvererzungen weisen weniger radiogene Bleiisotopenzusammensetzungen auf, als Bruch- und Scherzonen in denselben Vererzungen. Diese Zonen mit radiogenem, kaledonischem Blei weisen auch höhere Blei-, Zink- und Gold-Gehalte auf als die übrigen Teile der Vererzung, was andeutet, daß diese Metalle zusammen mit dem radiogenen Blei zu einem viel späteren Zeitpunkt in die Vererzung eingebracht worden sind. Die proterozoischen Vererzungen bewirkten die Metallausfällung aus Fluiden, die entlang von kaledonisch mobilisierten Verwerfungen und Scherzonen flossen.

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With 3 Figures     

Primitive and contaminated basalts from the Southern Rocky Mountains,U.S.A.

Basalts in the Southern Rocky Mountains province have been analyzed to determine if any of them are primitive. Alkali plagioclase xenocrysts armored with calcic plagioclase seem to be the best petrographic indicator of contamination. The next best indicator of contamination is quartz xenocrysts armored with clinopyroxene. On the rocks and the region studied, K₂O apparently is the only major element with promise of separating primitive basalt from contaminated basalt inasmuch as it constitutes more than 1 % in all the obviously contaminated basalts. K₂O: lead (> 4 ppm) and thorium (> 2 ppm) contents and Rb/Sr (> 0.035) are the most indicative of the trace elements studied. Using these criteria, three basalt samples are primitive (although one contains 1.7% K₂O) and are similar in traceelement contents to Hawaiian and Eastern Honshu, Japan, primitive basalts.Contamination causes lead isotope ratios, ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb, to become less radiogenic, but it has little or no effect on ⁸⁷Sr/⁸⁶Sr. We interpret the effect on lead isotopes to be due to assimilation either of lower crustal granitic rocks, which contain 5–10 times as much lead as basalt and which have been low in U/Pb and Th/Pb since Precambrian times, or of upper crustal Precambrian or Paleozoic rocks, which have lost much of their radiogenic lead because of heating prior to assimilation. The lack of definite effects on strontium isotopes may be due to the lesser strontium contents of granitic crustal rocks relative to basaltic rocks coupled with lack of a large radiogenic enrichment in the crustal rocks.Lead isotope ratios were found to be less radiogenic in plagioclase separates from an obviously contaminated basalt than in the primitive basalts. The feldspar separate that is rich in sodic plagioclase xenocrysts was found to be similar to the whole-rock composition for ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb whereas a more dense fraction probably enriched in more calcic plagioclase phenocrysts is more similar to the primitive basalts in lead isotope ratios.The primitive basalts have: ²⁰⁶Pb/²⁰⁴Pb 18.09–18.34, ²⁰⁷Pb/²⁰⁴Pb 15.5, ²⁰⁸Pb/²⁰⁴Pb 37.6–37.9, ⁸⁷Sr/⁸⁶Sr 0.704–0.705. In the primitive basalts from the Southern Rocky Mountains the values of ²⁰⁶Pb/²⁰⁴Pb are similar to values reported by others for Hawaiian and eastern Honshu basalts and abyssal basalts, whereas ²⁰⁸Pb/²⁰⁴Pb tends to be equal to or a little less radiogenic than those from the oceanic localities. ⁸⁷Sr/⁸⁶Sr appears to be equal to or a little greater than those of the oceanic localities. These ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁸Pb/²⁰⁴Pb ratios are distinctly less radiogenic and ⁸⁷Sr/⁸⁶Sr values are about equal to those reported by others for volcanic islands on oceanic ridges and rises.Publication authorized by the Director, U.S. Geological Survey     

Lead in archaeological samples: an isotopic study by ICP-MS

Inductively coupled plasma mass spectrometry (ICP-MS) has been used to measure the concentration and isotopic composition of Pb in archaeological human and animal skeletal remains, soil from a village site of the Omaha tribe (U.S.A.) and cosmetic pigments.Lead concentrations in human bones from the Omaha tribe vary between 4.8 and 2570 μg/g, with younger people having the highest concentrations. Lead concentrations in animal bones from an Omaha village vary between 0.6 and 3.7 μg/g, and those of three soil samples range between 18 and 21 μg/g. Lead concentrations found in human bones from Anasazi (Utah, U.S.A.) and Alta (Peru) populations vary between 0.7 and 3.2 μg/g.Isotope ratios of a reagent grade Pb(NO₃)₂ solutions were measured by thermal ionization mass spectrometry (TIMS), as well as by ICP-MS to provide laboratory reference materials. The accuracy of the ICP-MS measurements relative to TIMS for the standard solution were found to be within 0.02–0.31% for²⁰⁶Pb/²⁰⁴Pb, 0.02–0.55% for²⁰⁷Pb/²⁰⁴Pb, and 0.16–0.56% for²⁰⁸Pb/²⁰⁴Pb. The precision of measurements on artifacts was 0.42–0.65% for²⁰⁶Pb/²⁰⁴Pb and 0.41–0.62% for²⁰⁷Pb/²⁰⁴Pb, whereas the precision for the same ratios for the bones was 0.85–1.8 and 0.82–1.67%, respectively. For the cosmetic lead-bearing pigments, a precision of 0.07–0.15% was found for both²⁰⁶Pb/²⁰⁴Pb and²⁰⁷Pb/²⁰⁴Pb ratios. Lead isotope ratios of artifacts give a radiogenic Pb signature, of which are close to signatures from PbZn mines of the central U.S. region. Lead isotope ratios of the pigments give non-radiogenic Pb signatures. Lead isotope ratios of the bones differ from those of the artifacts, and although similar in isotopic ratio to the pigments, they are more scattered, suggesting potential mixing of Pb from different regions.     

Single-grain 207Pb206Pb and U/Pb age determinations with a 10-μm spatial resolution using the ion microprobe mass analyzer (IMMA)

Analytical techniques have been developed for using a secondary ion mass spectrometer, the ion microprobe mass analyzer (IMMA), to determine, in situ, ${}^{207}\text{Pb}{}^{206}\text{Pb}$ and U/Pb ages on approximately 10-μm areas of individual mineral phases containing relatively abundant radiogenic Pb isotopes. Standard samples of known age and U, Th and Pb contents, together with the Andersen-Hinthorne local thermal equilibrium (LTE) model for predicting ionization parameters are used to establish a semi-empirical relationship for correcting observed U/Pb intensities to atom ratios. Measurements of isotope standards show that mass fractionation corrections are not required and that the accuracy and precision of analysis are generally limited by Poisson counting statistics.Many U-rich accessory minerals yield spectra which consist only of Pb at masses 204, 206, 207 and 208; thus the measurement of ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages is accomplished by simply measuring the intensities of these peaks and the background. An excellent correspondence of the ages determined using the IMMA with those from more conventional techniques is demonstrated for terrestrial, lunar and meteoritic phases. For example, the following ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages were obtained from polished thin sections of crystalline lunar samples: 10047, 3.75 ± 0.05 (2σ) Ga; 14310, 3.96 ± 0.03 Ga; and 15555, 3.36 ± 0.06 Ga. From small U-rich phases in CaAl-rich inclusions in the Allende carbonaceous chondrite, seven ${}^{207}\text{Pb}{}^{206}\text{Pb}$ ages were obtained, averaging 4.60 ± 0.09 (2σ) Ga.A method of correcting low-resolution Pb-isotope data for molecular ion interferences in zircon and baddeleyite is presented and shown to be practical through the analysis of terrestrial and lunar samples.     

Metal sources of the large Nezhdaninsky orogenic gold deposit, Yakutia, Russia: Results of high-precision MC-ICP-MS analysis of lead isotopic composition supplemented by data on strontium isotopes

A collection of galena from the Nezhdaninsky gold deposit (62 samples), as well as galena from the Menkeche silver-base-metal deposit and the Sentyabr occurrence and K-feldspar from intrusive rocks of the Tyry-Dyby ore cluster have been studied using the high-precision (±0.02%) MC-ICP-MS method. Particular ore zones are characterized by relatively narrow variations of isotope ratios (no wider than σ_6/4 = 0.26%). Vertical zoning of Pb isotopic composition is not detected. Variation in Pb isotope ratios mainly depends on the type of mineral assemblage. Galena of the gold-sulfide assemblage dominating at the Nezhdaninsky deposit is characterized by the following average isotope ratios: ²⁰⁶Pb/²⁰⁴Pb = 18.472, ²⁰⁷Pb/²⁰⁴Pb = 15.586, and ²⁰⁸Pb/²⁰⁴Pb = 38.605. Galena from the regenerated silver-base-metal assemblage is distinguished by less radiogenic lead isotope ratios: 18.420, 15.575, and 38.518, respectively. In lead from the Nezhdaninsky deposit, the component, whose source is identified as Permian host terrigenous rocks, is predominant. The data points of isotopic composition of lode lead make up a linear trend within the range of μ₂ = 9.5-9.6. K-feldspar of granitic rocks has less radiogenic and widely varying lead isotopic composition compared to that of galena. The isotopic data on Pb and Sr constrain the contribution of Late Cretaceous granitic rocks as a source of gold mineralization at the Nezhdaninsky deposit. The matter from the Early Cretaceous fluid-generating magma chamber participated in the ore-forming system of the Nezhdaninsky deposit. The existence of such a chamber is confirmed by the occurrence of Early Cretaceous granitoid intrusions on the flanks of the Nezhdaninsky ore field. The greatest contribution of magmatic lead (～30%) is noted in galena from the silver-base-metal mineral assemblage. This component has isotopic marks characteristic of lower crustal lead: the elevated ²⁰⁸Pb/²⁰⁶Pb ratio relative to the mean crustal value and the lower ²⁰⁷Pb/²⁰⁴Pb ratio. Taken together, they determine a high Th/U ～ 4.0 in the source and μ₂ = 9.37–9.50. This conclusion is consistent with the contemporary tectonic model describing evolution of the South Verkhoyansk sector of the Verkhoyansk Foldbelt and the Okhotsk Terrane.